A spark ignition internal combustion engine for vehicular use is typically characterized by a four-stroke cycle of operation. Each piston of the engine reciprocally translates within a respective cylinder between a top dead center position and a bottom dead center position, as understood by those skilled in the art. During an intake stroke, the piston moves from its top dead center position to its bottom dead center position, thereby drawing air and fuel into the cylinder through an open intake valve. During a subsequent compression stroke, the piston moves from its bottom dead center position to its top dead center position, thereby compressing the air and fuel.
A spark plug generates a spark, causing the air and fuel in the cylinder to combust when the piston is at or near top dead center, generating pressure and forcing the piston to its bottom dead center position during an expansion stroke. After the expansion stroke, the piston returns to its top dead center position during an exhaust stroke, thereby forcing the exhaust gases out of the cylinder through an opened exhaust valve. In the Otto cycle, the pressure in the cylinder is higher than the exhaust manifold pressure at the time the exhaust valve is opened, and thus some of the energy produced by the combustion of the air and fuel is lost through the exhaust rather than transferred to the crankshaft via the piston.
An engine operating in the Atkinson cycle is characterized by significantly higher efficiency than an engine operating in the Otto cycle because the Atkinson cycle permits more expansion of the combustion gases prior to the opening of the exhaust valve, e.g., the pressure in the cylinder may be equal to the pressure in the exhaust manifold or atmospheric pressure when the exhaust valve is opened. However, the Atkinson cycle is characterized by a lower indicated mean effective pressure (IMEP), power density, and peak power than the Otto cycle.